PNNL

Abstract

The non-closure of surface energy balance remains an outstanding problem in eddy covariance (EC) measurements of land-surface fluxes of heat, water vapor, and CO2. Our data collected from an EC tower over a semi-arid sagebrush ecosystem indicate that under unstable atmospheric conditions, the non-closure becomes increased with the increasing instability, in consistent with many other studies. It is demonstrated here that changes in the structures of large-scale turbulent motions cause the increased non-closure as instability increases. Spectral analysis shows that the scales of turbulent motions dominating sensible and latent heat fluxes become enlarged with the increasing instability. Quadrant analysis is then used to reveal that the flux contributions from ejections remain nearly constant with the increasing instability, whereas the flux contributions from sweeps are reduced and their time fractions increase. Our results suggest that the increased non-closure of surface energy balance is largely associated with changes in turbulent structures as the instability increases.